Implemented validation on request header keys on Req.new

[octowombat]

Not sure if I put the logic in the best place but it's where I thought it made sense to me!

[wojtekmach]

Thank you for the PR. It looks good to me, I'm just still trying to figure out whether we should only do the checking on Req.new (and thus on Req.get, Req.post, etc too) or everywhere, including the lower-level Req.Request.* functions.

[wojtekmach]

@octowombat thank you for pursuing this. This gave me a lot to think about. I decided to:

1. always downcase header names
2. warn if they aren't downcased

This is done in #227.

My critical insight was if we ever want to change the behaviour (e.g. noop for maximum performance, raise, etc) it should be done by a compile-time configuration. That is, the __encode_header_name__/1 that I introduced in the PR would become a macro that would return argument as is in :noop mode, generate code that checks and warns at runtime in other mode, etc.

I'm sorry that I won't be merging your PR but it really was very influential, all the edge cases you explored in docs and tests.